What ever was used in the past doesn`t matter, it has to be accepted that the word will be found offensive and clarkson should not have even gone there.
You dont need to say it, just touching on the subject is obvious you will be too close to the mark and he should have shown better judgement.
What rap stars say and do is up to them but it does not make it acceptable for others to say it as well.
Anyone who thinks its ok try doing it in Brixton and see if you get home that night....... Be sensible eh.